It is desirable for the tires of a motor vehicle to remain properly inflated. When the tires of a motor vehicle are under-inflated or “low”, the tire rolling resistance increases and therefore the vehicle's fuel economy decreases. In addition, the vehicle's handling characteristics, such as steering and braking, may be adversely affected. Low tires also experience greater flexing than properly inflated tires, leading to excessive wear and a decrease in usable tire life. Accordingly, tire inflation should be monitored to ensure proper inflation levels.
To ensure proper tire inflation levels, numerous monitoring strategies have been developed. Such strategies include those that directly measure tire pressure or utilize effective rolling radius calculations to determine when the radius of one of the wheels varies. The generally employed principle of using the effective rolling radius relies on the fact that a wheel with a flat or low pressure tire has an incrementally smaller effective rolling radius than a properly inflated tire. Often, sensors are used to measure the rotation of each wheel wherein sensor measurements are related to the effective rolling radius. The tire inflation level may be determined based on the effective rolling radius.
For radial construction tires, the effective rolling radius may be weakly dependent on tire pressure. Therefore, determining tire inflation for radial construction tires is more difficult. A large “hoop tension” in the tire belt keeps the tire rolling radius almost constant with respect to tire inflation. For example, some tests indicate that a tire inflated to only 3 pounds per square inch (psi) may have a rolling radius approximately 0.9% smaller than if it were inflated to its nominal pressure, 30 psi. However, very accurate measurement of rolling radius has become economically feasible due to the enhanced dynamic range of modern 16-bit microprocessors commonly used in Anti-lock Braking Systems (ABS) and which read the wheel rotation sensors.
Another challenge in monitoring tire inflation by rolling radius relates to the fact that some tire characteristics have a larger influence upon wheel effective rolling radius than inflation pressure. Tire-to-tire manufacturing tolerances may vary the effective rolling radius by up to 0.4%. Also, during the tire break-in period, approximately the first 100 miles, the effective rolling radius typically may change up to 0.6%. Tread wear may also significantly change the effective rolling radius over the tire lifetime, up to 0.5%.
Vehicle operating conditions may also offset the effective rolling radius determinations of tire inflation status. These conditions are those which cause wheel slippage, those related to the use of a “space-saver” spare tire, and those related to speed. Generally speaking, maneuvers that result in even slight to moderate wheel slippage may cause the effective rolling radius to change by an amount greater than that to be caused by pressure variation alone. Such maneuvers include accelerating, decelerating using brakes, steering through sharp turns, and any combinations of these.
Vehicle operation at very low speeds (e.g., less than about 6 mph or 10 kph) may also offset the effective rolling radius determinations of tire inflation status. One reason is an increased likelihood of wheel slip due to acceleration, deceleration, and steering. This is because low speed operation is not a sustained operating point, but a transitional one during which the car is decelerating to stop, accelerating to normal driving speeds, or steering through sharp turns and corners. Also, at low speeds, the wheel rotation sensor signal drops to a very low amplitude level and may become “noisy” or nonexistent. The loss of signal integrity at low speed is a characteristic of the most widely used wheel rotation sensor technology. Therefore, it would be desirable to overcome many of the aforementioned disadvantages associated with monitoring tire inflation based on determinations of rolling radius.
To overcome some of the shortcomings associated with using rolling radius based determinations, sensors may be used to directly measure tire pressure of each tire. The direct pressure sensors may provide an effective alternative strategy for monitoring tire inflation as they are not subject to many of the factors that can offset determinations based on rolling radius. Although such sensors may facilitate accurate tire inflation monitoring, providing a sensor for each vehicle tire increases the cost of the monitoring system. Accordingly, it would be desirable to provide a strategy for monitoring tire inflation without the need for a sensor at each vehicle tire.
Therefore, it would be desirable to provide a strategy for monitoring vehicle tire inflation that overcomes the aforementioned and other disadvantages.